1. Field of the Invention
The present invention relates to a photomask used for production of a hologram element designed for use in an optical pickup device which is mounted in an optical disk recording/reproducing apparatus, a method for producing a hologram element using a photomask, and a hologram element.
2. Description of the Related Art
An optical disc such as compact disc (abbreviated as CD) or digital versatile disc (abbreviated as DVD) is capable of recording large volumes of information at high recording density, and thus finds a wider range of applications. For example, it is employed in an audiovisual system and a computer.
FIG. 15 is a perspective view showing an optical pickup apparatus 1 of conventional design. The optical pickup apparatus 1 is composed of a photodetector 2, a beam splitter 3, a collimator lens 4, an objective lens 5, and a light source 6. Light emitted from the light source 6 is reflected from the beam splitter 3, and is then converted through the collimator lens 4 into parallel light. Through the objective lens 5, the parallel light converges on an information recording surface of an optical disc 10. After being reflected from the information recording surface of the optical disc 10, the signal light carrying the information recorded on the optical disc 10 passes through the objective lens 5 and the collimator lens 4, and the beam splitter 3 so as to be incident on the photodetector 2.
In order to enhance the reproduction characteristics of the optical pickup apparatus which is mounted in an optical disc recording/reproducing apparatus for reading out information recorded on an optical disc, as much reproduction signal light as possible needs to be incident on the photodetector. The quality of the reproduction signal has hitherto been improved by employing a polarizing optical system. However, with recent improvements in the technology on disc drives and light detecting portions including light-receiving elements, it has become possible to obtain satisfactory reproduction characteristics even though signal light is not sufficiently intense. Besides, there has been an increasing demand for an apparatus both smaller in size and lower in cost. As a natural consequence of such a trend, an optical system of simpler structure has come into wider use instead of a polarizing optical system. As a representative example thereof, there has been known an integrated unit optical system that is constructed by combining a light source and a light detecting portion using a diffraction element.
FIG. 16 is a perspective view showing an optical pickup apparatus 11 employing an integrated unit optical system of another conventional design. The optical pickup apparatus 11 has a simple structure, which is roughly composed of an integrated unit 12 and an objective lens 13. In the optical pickup apparatus 11, the integrated unit 12 is arranged in such a way that the optical axis of the light emitted from the integrated unit 12 is aligned parallel to an information recording surface of an optical disc 10. Moreover, in the optical pickup apparatus 11, a collimator lens 14 is arranged in the area between the integrated unit 12 and the objective lens 13. Note that, in the optical pickup apparatus 11 employing such an integrated unit 12, a raising mirror may additionally be disposed in the area between the integrated unit 12 and the objective lens 13. The raising mirror serves to deflect the light emitted from the integrated unit 12 so as for the optical axis of the light converging on the information recording surface of the optical disc 10 to be perpendicular to the information recording surface.
The integrated unit 12 is composed of a hologram element 15, a laser element 17 acting as a light source, and a light-receiving element 18 acting as a light detecting portion. The hologram element 15 is provided with a hologram 16. The hologram 16 is divided into two regions, in each of which a diffraction grating is formed. Light emitted from the laser element 17 passes through the hologram 16 of the hologram element 15 and the collimator lens 14 in this order, and then converges on the information recording surface of the optical disc 10 through the objective lens 13. The light reflected from the optical disc 10 passes through the objective lens 13 and the collimator lens 14, and is then incident on the hologram 16 where the light is diffracted by the diffraction grating disposed in each of the hologram regions, whereby converted into first-order diffracted light. The first-order diffracted light is incident on a light detecting portion 18. The reflection light diffracted in one of the regions of the hologram 16 is used to detect pit data recorded on the optical disc 10, whereas the reflection light beam diffracted in the other region is used to detect pit data and a focus error signal.
Moreover, a tracking error signal is detected by exploiting the difference in intensity between the light beams diffracted in the respective regions of the hologram 16. In this case, if the diffraction gratings of the respective hologram regions are unequal in first-order diffraction efficiency (i.e. the ratio of the first-order diffracted light quantity to the whole light quantity), an offset is caused in the tracking error signal, and thus the optical pickup apparatus fails to achieve the following of the track on the optical disc by a light beam. As a result, the pit data cannot be detected properly. To overcome this problem, the hologram element has conventionally been so designed that the first-order diffraction efficiency ratio between the diffraction gratings of the respective hologram regions is kept in the range of 0.9 to 1.1. The first-order diffraction efficiency of the diffraction grating constituting the hologram is determined according to the groove depth of the diffraction grating and a duty ratio which is expressed as the ratio of the groove opening width to the grating interval. However, it is difficult to properly control the groove depth and groove opening width of the diffraction grating.
FIGS. 17A to 17F are sectional views showing the process steps for producing the hologram in accordance with a photolithography method. At first, in the substrate cleaning process shown in FIG. 17A, a surface of a glass substrate 21 is subjected to cleaning. Then, in the resist coating process shown in FIG. 17B, a resist 22, i.e. a photosensitive body, is applied to the surface of the glass substrate 21 in accordance with a spin-coat method, followed by a baking finish to volatilize the solvent.
In the exposure process shown in FIG. 17C, a photomask is brought into intimate contact, via the resist 22, with the glass substrate 21. The photomask has a fine diffraction-grating pattern for constituting the hologram. Then, ultraviolet rays are applied thereto to form the fine diffraction-grating pattern on the resist 22. Thereafter, in the etching process shown in FIG. 17D, the glass substrate 21 having the resist 22, on which the fine diffraction-grating pattern is formed through the exposure process, is subjected to dry etching using a reactive ion etching (RIE for short) device. Used as etching gas here is for example tetrafluoromethane (chemical formula: CF4) gas or trifluoromethane (chemical formula: CHF3) gas.
In the ashing process shown in FIG. 17E, the residual resist 22 remaining on the glass substrate 21 is removed by use of solvent, or by performing an ashing removal operation in an oxygen-gas atmosphere; wherefore concavities and convexities corresponding to the resist pattern are formed on the glass substrate 21. In the separation process shown in FIG. 17F, a plurality of holograms formed on the glass substrate 21 are separated to realize the final necessary configurations.
FIG. 18 is a flow chart showing the procedure for controlling a diffraction efficiency ratio as observed in the production of the hologram in accordance with the photolithography method. At Step u0, diffraction efficiency ratio control is started, and the procedure proceeds to Step u1. At Step u1, upon changing of the photomask, the photomask is brought into intimate contact with the glass plate. Then, the procedure proceeds to Step u2. At Step u2, the first-order diffraction efficiency of each hologram region is measured three times with varying exposure time. Then, the procedure proceeds to Step u3.
At Step u3, whether the first-order diffraction efficiency ratio is kept in the range of 0.9 to 1.1 or not is checked. The first-order diffraction efficiency is at its peak when the integral of the quantity of light reaches a certain value at which the diffraction grating has a duty ratio of 0.5. Thus, firstly determined is an optimum value for the light-quantity integral at which the measured first-order diffraction efficiency in each region is at the maximum. Then, a first-order diffraction efficiency ratio corresponding to the optimum light-quantity integral is obtained. The light-quantity integral is the product of the light quantity and the exposure-time duration. Whether the first-order diffraction efficiency ratio is kept in the range of 0.9 to 1.1 or not is checked, if not, the procedure returns to Step u2, where the exposure condition is changed once again. The operations in Steps u2 to u3 are repeated until the first-order diffraction efficiency ratio falls in the range of 0.9 to 1.1.
If, at Step u3, the first-order diffraction efficiency ratio is judged to fall in the range of 0.9 to 1.1, an optimum exposure condition is determined, and the procedure proceeds to Step u4. At Step u4, the hologram is produced in quantity in accordance with the determined exposure condition, and the procedure proceeds to Step u5. At Step u5, the procedure for controlling the diffraction efficiency ratio comes to an end. In this way, by performing optimum light exposure amount control whenever the photomask is changed, the first-order diffraction efficiency ratio between the two hologram regions can constantly be kept in the range of 0.9 to 1.1.
In addition to the above-described glass element, an element made of ultraviolet-ray-setting resin may also be used as the hologram element for use in such an optical system (refer to Patent Documents 1 and 2, for example). By using such an ultraviolet-ray-setting resin element, in contrast to the case of using a conventional glass element, not only it is possible to prepare the required material at lower cost, but it is also possible to adopt, as a manufacturing method, the so-called photopolymer method (2P method for short) which is excellent in productivity. This helps reduce the production cost of the hologram element.
FIGS. 19A to 19C are sectional views showing the process steps for producing the hologram in accordance with the photopolymer method. At first, as shown in FIG. 19A, a pair of stampers 23A and 23B and a resin substrate 24 are prepared. The stampers 23A and 23B are each constructed by forming a diffraction-grating pattern on a glass plate in accordance with the above-described photolithography method. The stamper 23A is arranged with its one surface carrying the diffraction-grating pattern facing with one surface of the resin substrate 24. The stamper 23B is arranged with its one surface carrying the diffraction-grating pattern facing with the other surface of the resin substrate 24. Then, ultraviolet-ray-setting resin 25 is applied to the one surface of the resin substrate 24. The ultraviolet-ray-setting resin 25 is also applied to that surface of the stamper 23B which carries the diffraction-grating pattern. As a resin material used to form the resin substrate 24, a transparent resin material is desirable that is resistant to degradation when irradiated with laser light. Specifically, acrylic resin is commonly used.
Subsequently, as shown in FIG. 19B, the stampers 23A and 23B are each brought into abutment with the resin substrate 24, followed by pressurizing as desired, so that the ultraviolet-ray-setting resin 25 is sufficiently spread across the surface of the resin substrate 24 under the pressure. Then, ultraviolet rays are applied thereto to cure the ultraviolet-ray-setting resin 25. After that, as shown in FIG. 19C, the stampers 23A and 23B are removed from the resin substrate 24 with the ultraviolet-ray-setting resin 25, thus forming a diffraction grating on the resin substrate 24.
The diffraction grating of the hologram element is realized in accordance with the conventional method thus far described. Here, the photomask in use is so designed that the respective hologram regions differ from each other in grating interval and curvature. This allows the signal light incident on the hologram to converge and enter the individual light-receiving regions formed on the light-receiving element 18 shown in FIG. 16. Moreover, the photomask is so designed that, when the grooves have the same depth, the hologram regions each have a duty ratio of 0.5 at which the highest first-order diffraction efficiency is obtained.
The related art is disclosed in Japanese Unexamined Patent Publications JP-A 10-187014 and JP-A 10-254335.
In general, the diffraction grating of the hologram is coated with a reflection preventive film to maximize the usability of laser light output. In this case, however, if the hologram element is produced using the above-described photomask designed such that the hologram regions each have a duty ratio of 0.5, the first-order diffraction efficiency ratio or, in the case of mass production, the mean value of the first-order diffraction efficiency ratio, may possibly be deviated from the optimum value: 1.0, if anything, fall outside the range from 0.9 to 1.1.
FIG. 20 is a graph showing the relationship between the first-order diffraction efficiency and light exposure amounts as observed in each of the two regions having different diffraction grating intervals. In FIG. 20, the solid line L10 represents the relationship between the first-order diffraction efficiency and light exposure amounts as observed in the first region having a grating interval of Δ, whereas the long dashed double-dotted line L11 represents the relationship between the first-order diffraction efficiency and light exposure amounts as observed in the second region having a grating interval of δ which is shorter than the grating interval Δ of the first region.
According to the photolithography method, as seen from the solid line L10 and the long dashed double-dotted line L11, the larger the light exposure amount, the higher the first-order diffraction efficiency can be. The first-order diffraction efficiency is at the maximum when the light exposure amount reaches a certain level. After that, as the light exposure amount is increased, the first-order diffraction efficiency is decreased. Moreover, as seen from the solid line L10 and the long dashed double-dotted line L11, the variation of the first-order diffraction efficiency with respect to the variation of the light exposure amount is more remarkable in the second region having a shorter grating interval than in the first region having a longer grating interval. The first and second regions can be equal in first-order diffraction efficiency, that is, the first-order diffraction efficiency ratio between the first and second regions can be given as 1.0: the optimum value, only when the light exposure amount reaches a certain level where the duty ratio is given as 0.5, that is, the first and second regions each have the maximum first-order diffraction efficiency. When the light exposure amount is at a level other than the aforementioned level, the region having a shorter grating interval has lower first-order diffraction efficiency.
FIG. 21 is a graph showing the relationship between the first-order diffraction efficiency and light exposure amounts found before and after a reflection preventive film is vapor-deposited, as observed in each of the two regions having different diffraction grating intervals. In FIG. 21, the dotted line L12 represents the relationship between the first-order diffraction efficiency and light exposure amounts found before vapor-deposition of a reflection preventive film, as observed in the first region having a grating interval of Δ; the solid line L13 represents the relationship between the first-order diffraction efficiency and light exposure amounts found after vapor-deposition of a reflection preventive film, as observed in the first region; the long dashed dotted line L14 represents the relationship between the first-order diffraction efficiency and light exposure amounts found before vapor-deposition of a reflection preventive film, as observed in the second region having the grating interval of δ which is shorter than the grating interval Δ of the first region; and the long dashed double-dotted line L15 represents the relationship between the first-order diffraction efficiency and light exposure amounts found after vapor-deposition of a reflection preventive film, as observed in the second region.
In general, a reflection preventive film is formed on the diffraction grating by stacking dielectric films in layers, in accordance with the vapor-deposition method. However, such a vapor-depositing process adversely affects the ideal steric structure of the diffraction grating fabricated by means of photolithography, resulting in a decrease in the first-order diffraction efficiency of the first and second regions. Specifically, as shown in FIG. 21, in the first region, the first-order diffraction efficiency changes from a state as indicated by the dotted line L12 to a state as indicated by the solid line L13, whereas in the second region, the first-order diffraction efficiency changes from a state as indicated by the long dashed dotted line L14 to a state as indicated by the long dashed double-dotted line L15. As seen from FIG. 21, the second region having a shorter grating interval undergoes a sharper decrease in the first-order diffraction efficiency. As a result, there occurs a disparity in first-order diffraction efficiency between the two regions, and thus the first-order diffraction efficiency ratio is greatly deviated from the optimum value: 1.0.
In a hologram element designed for use in an optical media reproducing apparatus for reading and writing an optical medium requiring writing of information, such as a CD Recordable (abbreviated as CD-R), the first-order diffraction efficiency is low, and variation in the first-order diffraction efficiency has a profound influence on the reading and writing on the optical medium.